Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a blowdown portion and scavenging portion of exhaust gases. The timing of the second set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, in the engine system described above, a composition of gas recirculated to the intake may be more complex than a traditional EGR system comprising a single exhaust manifold or a system that does not recirculate increased volumes of blowthrough air. Whereas recirculated gas in traditional EGR systems is entirely comprised of burnt gas, the gas recirculated through the split exhaust engine may include varying portions of burnt gas, fresh air, and pushback (e.g., unburnt or non-combusted) fuel. Timing adjustment of engine operations such as fuel injection, spark advance, and intake and exhaust valve actuation timings based on an assumed EGR gas composition of traditional EGR systems may result in degraded engine performance in the split exhaust engine. Thus, a method to determine the composition of the recirculated gas in the split exhaust engine including blowthrough based on a unique configuration of the engine is desirable to estimate an EGR dilution rate (e.g., a dilution value or rate of the gases recirculated to the intake passage) for accurate engine control.
In one example, the issues described above may be addressed by a method for determining a dilution rate of gas recirculated from a first set of exhaust valves to an intake passage based on a timing of the first set of exhaust valves while flowing exhaust gas from a second set of exhaust valves to a turbocharger turbine and not to the intake passage, where each cylinder includes one valve from each of the first and second set of exhaust valves. As one example, a feedforward model that evaluates a total exhaust gas recirculation (EGR) mass flow, a temperature of the EGR gas, a mass of burnt gas, an airmass due to blowthrough, a fuel mass due to blowthrough, and a burnt gas fraction during engine transient events, as functions of engine speed and load, is used to estimate the EGR dilution rate. By accounting for the contributions of each component of the gas mixture (including fresh air and recirculated gas, or scavenge gas) circulated through the split exhaust engine and an effect of a pressure differential across an intake region, an estimation of the EGR rate is tailored to an architecture of the split exhaust engine and may allow fuel injection and spark timing to be adjusted to increase an efficiency and power output of the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.